It has been widely known to use a polypropylene resin composition, which is obtained by compounding an ethylenic thermoplastic elastomer component such as an ethylene-propylene copolymer, ethylene-butene copolymer, etc. and an inorganic filler such as talc, etc. to a polypropylene resin, for producing automotive parts. Further, it has been proposed to improve moldability, mechanical properties and appearance by suitably selecting the kind of polypropylene resins, various rubber components and inorganic fillers depending on the purpose.
However, the materials containing these inorganic fillers generally have a tendency to cause poor molded appearance like tiger stripes, i.e., so-called flow marks, which spoils designs of the molded articles obtained. Meanwhile, as a method for improving such poor molded appearance, it is proposed to use a material having a wide molecular weight distribution in JP-A-2000-86837 and JP-A-6-248155 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.). Further, in JP-A-9-176406, JP-A-9-194646 and JP-A-9-124736, a material obtained by using a propylene-ethylene block copolymer having a specific structure is proposed for improving flow marks.
However, the use of such a material having a wide molecular weight distribution can spoil weld appearances, though it improves the problem of flow marks. Thus, further improvement is required now.
JP-A-6-329737 discloses a propylene block copolymer which comprises a specific propylene component and a component consisting of propylene and other α-olefins. This copolymer is described to be easily molded and processed, to have excellent mechanical properties such as rigidity and tensile strength, transparency as well as surface appearance, and to raise molding rate.
Moreover, JP-A-7-53828 discloses a polypropylene resin composition comprising an ethylene-propylene block copolymer, a hydrogenated block copolymer, an elastomer and talc. The composition is described to be suitable for large-size automotive parts which are light and excellent in rigidity, heat distortion resistance, impact resistance, dimensional stability as well as appearance, and of which molding cycles are short. In JP-A-11-181025, a propylene-ethylene-l-butene polymer having a specific MFR, Q value, etc. is disclosed as a propylene type resin which is excellent in flexibility, transparency, heat resistance and moldability, and is improved in bleeding to the surfaces of its molded articles.
Even with these technologies, however, there has been a problem of the tendency to cause flow marks.
With an intention to solve this problem, the invention aims at providing a composition which is excellent in moldability, can give good appearance when molded, and therefore is suitable for exterior parts of automobiles such as bumpers, rocker panel, side crudding, over fenders, etc.